Known recording materials for recording information using a light beam of high energy density, such as laser, include thermal recording materials in addition to light sensitive materials using silver salts.
In the thermal recording material, the recording layer has a high optical density and absorbs an irradiated light beam of high energy density causing a local temperature increase which causes thermal deformation such as melting, evaporation and aggregation. Accordingly, the irradiated portions are removed and change in optical character such as by forming a difference in optical density from the unirradiated portions whereby the light information is recorded. Such a thermal recording medium is desirable because the recording material does not need processings such as development and fixing. Further, use of such a recording material does not require work in a dark room since it is not sensitive to ordinary room light. In addition, the recording material provides an image having a high contrast, and it is possible to add information to the recording material.
In general, such a thermal recording material is frequently recorded by converting information to be recorded into electric time series signals and scanning the recording material with a laser beam the intensity of which is modulated according to the signals. This method is desirable because recorded images are obtained at real time.
Since thermal recording material has the unique features as described above, the application of the recording material to various uses such as, for example, substitution for lithographic printing films, facsimile recording materials, photomasks for integrated circuit (IC), microfilms, etc., has been attempted and a part of these applications are now used in practice. Because of having various uses and applications, such recording materials have been actively developed and investigated by many technical organs and various materials such as metals, plastics, dyes, etc., have been proposed as the materials for the recording layers. These materials are practically described in, for example, M. L. Levene et al., Electron Ion and Laser Beam Technology, the record of the 11th Symposium (1969), Electronics, page 50 Mar. 18, 1968); D. Maydan, The Bell System Technical Journal, Vol. 50, page 1761 (1971); C. O. Carlson, Science, Vol. 154, page 1550 (1966), U.S. Pat. Nos. 4,188,214, 4,291,119, 4,216,501, 4,233,626 and 4,188,214, and British Patent No. 2,026,346.
In these known techniques, it can be said that every effort has been made to improve the recording sensitivity of the recording materials. Efforts made toward improving the sensitivity of recording materials when using thin metal layers are summarized below.
Recording materials using a thin layer of a metal such as Bi, Sn, In, Al, Cr, etc., have excellent properties such as high resolving power and high contrast. However, many of these recording materials have a light reflectance to laser light of higher than 50% and thus the energy of laser light cannot be effectively utilized by these recording materials. Therefore, the light energy required for recording is large and a laser light source of a high output power is required for recording highspeed scanning, thereby the recording device becomes larger and expensive.
Accordingly, various recording materials having a high recording sensitivity have been investigated. For example, U.S. Pat. No. 3,560,994 describes a recording layer composed of Se, Bi and Ge layers. In this case, the layer of Ge reduces the light reflectance of the layer of Bi to irradiated light, and also the layer of Se is an easily evaporating layer and accelerates the the thermal deformation of the Bi layer which is the main recording layer, and the resulting recording layer can record a light information with less energy than when a layer of Bi is used alone. Furthermore, a layer for reducing or preventing light reflection is described in U.S. Pat. Nos. 4,335,198 and 3,665,483. Also, a layer for reducing the thermal conductivity provided between a recording layer and a support is described in Japanese Patent Application (OPI) No. 12,637/'75 and U.S. Pat. No. 3,911,444. Furthermore, a recording layer composed of a certain kind of metal sulfide, metal fluoride, or metal oxide and a metal as a mixture thereof or as a double layer of them is described in U.S. Pat. Nos. 4,188,214 and 4,415,650. Still further, a recording layer composed of a mixture of an inorganic material and an organic material is also disclosed in Japanese Patent Application (OPI) No. 5742/'79.
As summarized above, various efforts have been made with respect to high sensitization alone and some of such recording materials have been improved to the stage of practical usefulness. The systems which use these recording materials and the circumferential techniques thereof have also progressed in regard to not only the recording materials used for new applications but also the recording materials used for the foregoing conventional purposes. Better properties continue to be required for both new and old applications. In particular, when using a thermal recording material for a new application, such as an optical disc memory, the requirement for better properties is very strong and it is substantially difficult to meet these requirements using conventional recording materials as described above.
The main properties of recording materials required for an optical disc memory are as follows:
(1) the recording material has a high sensitivity enabling high-speed data writing,
(2) reflection reading or information is preferred for simplifying the optical system and thus the recording layer must have a high light reflectance for realizing the above requirement.
(3) the recording material has chemical stability capable of stably retaining the recorded information for a long period of time (i.e., archival property of at least 10 years),
(4) the recording material has a high resolving power capable of making high density recording,
(5) the recording material gives recorded bits of good form for increasing the S/N ratio at reading (for the purpose, lack of uniformity in granularity of recording layer, etc., is undesirable),
(6) the recording material is excellent in producticn aptitude (for example, the evaporation speed during vapor deposition is stable, and also decomposition, etc., do not occur during vapor deposition), and
(7) materials used for the recording material are nontoxic.
Many other properties are also desired or required for such recording materials in addition to the foregoing properties which are not listed above. In the foregoing properties, the property (1), i.e., the high sensitivity generally conflicts with the property (2), the high reflectance. Also, it is generally difficult to satisfy the properties such as preservability, nontoxicity, etc., using materials having a relatively high sensitivity, such as a thin Te layer. Also, for high sensitivity, a low-melting metal such as In, Sn, etc., is considered to be preferred but in the case of a one component thin layer of such a metal, the thin layer is liable to take a island-like structure and hence it is difficult to satisfy the property (5) with such a material.
As described above, the materials used for optical disc memory are required to have the very best possible properties and in fact, materials simultaneously satisfying such properties as high sensitivity, long preservability and nontoxicity have not yet been discovered.
Thermal recording materials having a recording layer composed of a metal and a non-metallic material (metal sulfides, metal fluorides or metal oxides) formed by simultaneous vapor deposition of the materials (as disclosed in U.S. Pat. Nos. 4,188,214, etc.) have considerably improved properties with respect to (1) to (7) above. However, such a recording layer is not yet capable of exhibiting fully satisfactory properties. Materials capable of fully satisfying the requirements of both high sensitivity and storage stability remain to be found. Japanese Patent Application (OPI) No. 124134/81 discloses an optical recording medium for use in optical disk memories, obtained by simultaneous deposition of In and SiO.sub.2 having high chemical stability on a substrate such that SiO.sub.2 is deposited in an amount of 40 to 60% by volume of the resultant deposited mixture. This recording medium, however, still has the disadvantage that the energy of the laser beam to be used for the recording is not sufficiently low and the speed of the recording is not sufficiently high.